1. Field of the Invention
The invention relates to a process and to an apparatus for obtaining cambered and/or glazed glass sheets usable as car windows. It more specifically relates to a process according to which the glass sheets are reheated flat in a traversing furnace, brought into a cambering station below a shaping upper member, raised and applied by suction to said upper member, and deposited on a lower member on which they are passed into a cooling, and in particular tempering, station.
2. Description of the Related Art
Great importance is attached in the production of car windows to obtaining the maximum optical quality, which obviously requires the absence of gripper marks and scratches. With respect to an even more severe quality standard, this also assumes the absence of optical defects due to the heat treatment undergone by the glass sheet. A particular example of such a defect is that resulting from the deformations of the glass sheet caused by its non-uniform support, e.g., by rollers, between which the glass sheet raised to a high temperature may sag under its own weight. The resulting undulations do not really produce contour defects, which is in accordance with what is required from the macroscopic standpoint, but a sight seen through the glass plate is deformed.
These undulations are produced in the final part of the furnace and/or in the cambering station when the temperature of the glass is at its maximum. In order to prevent these, it is indispensable to keep the temperature below a threshold, where the deformation reaction kinetics are still sufficiently low compared with the glass sheet travel speed. The present inventors have found that this condition is generally satisfied if the temperature of the glass sheet does not exceed approximately 630.degree. C. However, such a temperature is inadequate at least in two possibly cumulative cases, namely where the glass sheet must be bent in accordance with a small radius of curvature and/or the glass sheet surface is totally or partly covered by a glazing composition.
In the first case, an excessively cold glass may lead to the breakage of the glass plate or window, whose viscosity does not make it possible to relax the stresses arising during cambering, except when proceeding extremely slowly, which is not compatible with the requirements of industrial production. It is certainly known from the art to bring about a very localized superheating or overheating by spreading over the glass at its entry into the furnace a powder of the carbon black type and/or by equipping the furnace with additional heating means travelling parallel to the glass (cf., e.g., U.S. Pat. No. 4,441,987 or DE-A-3742481). However, either these means must only act on a small part of the glass plate surface, or precisely the same undulation problems will occur on leaving the furnace.
In the second case, an inadequate temperature does not permit the complete baking of the glazing composition. When the glass sheet is brought into contact with the upper member, the latter or more precisely its coating, e.g., based on refractory fiber paper, may be stained by the still liquid composition and may dirty the following glass plates. Independently of the curvature which may or may not have to be given to the glass sheet, it is once again preferable to work with glass sheets whose temperature is, e.g., above 650.degree. C., which leads to the aforementioned optical defects.